1. Field of the Invention
This invention relates to a technique of removing metal contained in a solution and, more specifically, it provides a method of and an apparatus for removing metal as well as a surface active agent (surfactant) having chelating ability and used suitably for such method and apparatus. In addition, this invention relates to a technique of separating the removed metal from the solution. Here, the term "removal of metal from a solution" refers to "chelation of metal" and does not necessarily refer to "separation of metal from a solution".
2. Description of the Prior Art
Japanese Laid-Open Patent Publication No. 2-274900 discloses a process of separating metal ions from a solution. This technique employs a chelating agent which chelates metal ions to generate insoluble matter in the solution. More specifically, in this technique, the chelating agent noted above is added to the solution containing metal ions, followed by filtering out the insoluble matter to separate metal ions from the solution. The chelating agent used here does not contain any hydrophilic group because it is necessary to generate insoluble matter after chelating metal ions.
The technique in which the chelating agent free from hydrophilic group is used to chelate metal ions and generate insoluble matter, thereby separating metal ions from the solution, has the following problems. One of the problems is that the insoluble matter that is generated by the chelating agent is usually very fine in grain size and is difficult to be filtered out. Another problem is posed when re-using the solution. That is, during re-use of the solution which contains chelating agent added excessively for metal removal, insoluble matter is generated. For example, during washing of a plated product, metal ions are dissolved in the washing solution. If this washing solution is re-used without any processing, the metal ions therein are attached to a plated product. For this reason, it is necessary to recover the washing solution and remove metal ions therein. At this time, it is difficult to add an amount of chelating agent which is neither excessive nor insufficient with respect to the amount of metal ions in the solution. In the case of the Japanese Laid-Open Patent Publication No. 2-274900 noted above, it is recommended to add double the theoretically necessary amount of the chelating agent to increase the metal ion capturing factor. In such case, it is inevitable that ligands which do not chelate metal ions remain in the washing solution supplied for re-use. Consequently, metal ions dissolved in the washing solution are progressively chelated by ligands which have not chelated metal ions, thus resulting in the generation of insoluble matter during the washing.